HISTOLOGY 



31 



The stem of Panicum fluitans is round in outline in the transverse 

 section and has a large cavity. Just close to the cavity and 

 separated from it by only one or two parenchymatous cells are 

 found vascular bundles forming a series. Outside this series of 

 bundles lies a sclerenchymatous band which is wavy, following 

 the lower edges of the large air-cavities. One series of bundles 

 is connected with this sclerenchymatous ring. The air-cavities 

 are large and uniform and are separated by bands of parenchy- 

 matous cells. In each of these bands lies a vascular bundle on the 

 upper side near the periphery. Sometimes we find, especially in 

 young stages, diaphragms of stellulate cells stretched across the 

 air-cavities. Later as the stem matures these disappear and the 

 cavities become conspicuous. (See figs. 42 and 43.) 



Structure of the root. — As already stated the roots of grasses 

 conform to the monocotyledonous type, but the variations met with 

 in their structure are not so great as in the case of the stem. The 

 root-tips are protected by root-caps, and the actual tip of the root 

 is very distinct in the roots of all grasses and it can be seen very 

 clearly in a longitudinal section of the root. The actual tip of the 

 root is sharply distinct from the root-cap as there are two distinct 

 sets of cells, one giving rise to the root-tip and the other to the 

 root-cap. 



The young root-tips are always free from root-hairs, and they 

 are confined to the portions behind the root-tips. The extent of 



FIG. 44. — Transverse section of a part of the root of Pennisetum cenc^roides. x ico 



1. Root-hair; 2. piliferous layer ; 3. cortex; 4. enciodermis ; 5. pericycle ; 

 6. xylem ; 7 phloem. 



the root-hair region will vary according to the vigour and develop- 

 ment of the roots and the nature of the soil. The root-hairs are 

 mere protrusions of the cells of the outermost layer of the cortex 

 of the root and this layer is called the piliferous layer. 



